Prostate cancer is the second leading cause of cancer-related deaths among men in the United States, underscoring the urgent need to improve overall outcomes. Radiation therapy is used extensively, but many tumors show resistance and new strategies are needed to increase tumor radiosensitivity. Clinical trials have demonstrated that androgen deprivation plus radiation therapy improves disease-free and overall survival versus radiation alone in prostate cancer patients with high-risk disease. However, relatively little is known about the mechanistic basis of these effects, and local failure following such a regimen remains common and is associated with progressive metastatic disease. Prostate tumors have hypoxic regions that are sites of increased hypoxia-inducible factor 1-alpha (HIF1?) transcription. HIF1? enhances androgen receptor (AR) signaling and together these tumor survival factors promote the onset and progression of prostate cancer. Further, AR and HIF1? are components of a signaling network that supports therapeutic resistance. This project seeks to understand the mechanism of AR and HIF1? degradation by the E3 ligase CHIP and to determine whether this degradation can be exploited to enhance the efficacy of IR by radiosensitizing prostate tumors. Our new preliminary data presented here establish that 1) signaling activated by the naturally occurring estrogen metabolite 2-methoxyestradiol (2ME) induces phosphorylation of the E3 ligase CHIP and degradation of both AR and HIF1?; 2) both CHIP phosphorylation and degradation of AR and HIF1? are dependent on Aurora A kinase (AurA) activity; and 3) CHIP knockdown enhances prostate tumor growth and radioresistance in xenografts. Based on these results we hypothesize that activation of CHIP by phosphorylation enhances radiation-induced prostate cancer cell death by targeting AR and HIF1? for degradation, thereby altering expression of genes critical for survival and radioresistance. To test our hypothesis we will: 1)Demonstrate that phosphorylation activates CHIP-mediated ubiquitination and degradation of AR and HIF1? in human prostate cancer cells2) 2: Determine the molecular basis for CHIP-mediated radiosensitization of prostate cancer cells, and characterize the signaling pathways involved through transcriptome analysis. 3): Determine if radiosensitization of prostate cancer cells by 2ME is mediated by the Aurora- CHIP-AR/HIF1? axis using genetically engineered mouse models.